1. Field of the Invention
The present invention relates to a voltage measuring device, in particular, including: a plurality of voltage measuring members respectively provided at blocks composed of series-connected unit cells made of secondary batteries composing an assembled battery for measuring voltages across the unit cells composing the blocks; and a control member for receiving measured results of the voltages across the unit cells via communication with the voltage measuring members.
2. Description of the Related Art
In these days, a hybrid vehicle (hereafter referred to as HEV) which uses both an engine and an electric motor becomes widely used. This HEV includes a low voltage battery about 12 V for starting the engine, and a high voltage battery for driving the electric motor. This high voltage battery is made of series-connected unit cells. The unit cell is a secondary battery such as nickel hydride battery or lithium battery.
After repeating charge and discharge cycles of the high voltage battery, a voltage across each unit cell, namely, a state of charge (SOC) is varied. With regard to the charge and discharge of the battery, in view of endurance of the battery and safety ensuring, when the unit cell having the highest SOC reaches an upper limit, the charge is stopped, and when the unit cell having the lowest SOC reaches a lower limit, the discharge is stopped. Accordingly, when the SOCs of the unit cells are varied, an enabled capacity of the battery is substantially reduced. Therefore, in the HEV, so called assistance and regeneration of the battery become insufficient, and a moving performance and mileage are reduced. Consequently, for equalizing the SOCs of the unit cells, the voltages across the unit cells should be measured.
FIG. 6 shows a conventional voltage measuring device for measuring voltages across the unit cells composing the high voltage battery (Patent Document 1, 2). Reference number BL indicates a low voltage battery. As shown in FIG. 6, for example, the low voltage battery BL is composed of one secondary battery.
Further, reference numeral BH indicates a high voltage battery. The high voltage battery is used as a battery for the electric motor of the HEV. The electric motor as a load is connected to both ends of the high voltage battery as necessary, and an alternator as a charger is connected to both ends of the high voltage battery as necessary.
The high voltage battery BH is divided to blocks B1 to Bn (n: arbitrary integral number). Each block of B1 to Bn is, for example, composed of two unit cells C11 to Cn2. The voltage measuring device includes high voltage measuring circuits 11 to 1n as a high voltage measuring member, and a low voltage system control circuit 30 as a controlling member. The low voltage system control circuit 30 is powered by the low voltage battery BL and controls the high voltage measuring circuits 11 to 1n. The high voltage measuring circuits 11 to 1n are provided corresponding to the blocks B1 to Bn. The high voltage measuring circuits 11 to 1n are powered by the unit cells C11 to Cn2 of corresponding block B1 to Bn from among the blocks B1 to Bn, and measures the voltage across the unit cells C11 to Cn2 of the corresponding block B1 to Bn. Namely, cathodes of corresponding blocks B1 to Bn are ground levels and the high voltage measuring circuits 11 to 1n have different ground levels. Thus, a device having a low breakdown voltage can be used for composing the high voltage measuring circuits 11 to 1n. 
The high voltage measuring circuit 11 is connected to the low voltage system control circuit 30 via a transmitting line LT1 and a receiving line LR1. A transmission isolation device DT such as a photo-coupler is provided on the transmitting line LT1. Owing to this transmission isolation device DT, while the high voltage measuring circuit 11 is isolated from the low voltage system control circuit 30, the low voltage system control circuit 30 can send a measuring order to the high voltage measuring circuit 11. Further, a reception isolation device DR such as a photo-coupler is provided on the receiving line LR1. Owing to this reception isolation device DR, while the high voltage measuring circuit 11 is isolated from the low voltage system control circuit 30, the low voltage system control circuit 30 can receive a measured result from the high voltage measuring circuit 11.
Further, the high voltage measuring circuits 11 to 1n are serially connected to each other via a transmission line LT2 and a reception line LR2. The high voltage measuring circuit 11 sends the measuring order received from the low voltage system control circuit 30 to the high voltage measuring circuits 11 to 1n via the transmission line LT2. The high voltage measuring circuits 11 to 1n send the measured result to the high voltage measuring circuit 11 via the reception line LR2. The high voltage measuring circuit 11 sends the measured result received from the high voltage measuring circuits 11 to 1n via the reception line LR2 to the low voltage system control circuit 30.
For example, after receiving the measured results of the unit cells C11 to Cn2 corresponding to an ignition off, the low voltage system control circuit 30 sends a sleep order to the high voltage measuring circuits 11 to 1n via the transmitting line LT1. In this sleep mode, the high voltage measuring circuits 11 to 1n stop a communication with the low voltage system control circuit 30 to reduce a dark current at the ignition off of a vehicle. Further, when the low voltage system control circuit 30 provide an awakening signal to a not-shown awakening terminal, the sleep mode of the high voltage measuring circuits 11 to 1n is exited even while a communication function does not work.
Next, a structure to send the awakening signal to respective high voltage measuring circuits 11 to 1n will be explained. In the voltage measuring device, isolation devices D1 to Dn are provided corresponding to the blocks B1 to Bn. Each of isolation devices D1 to Dn includes a light emitting device LE and a photo receiver LD. One end of the light emitting device LE is connected to the low voltage battery BL via a resistor R1, and the other end of the light emitting device LE is connected to the low voltage system control circuit 30. Further, one end of the photo receiver LD is connected to an anode V1 to Vn of corresponding blocks B1 to Bn via a resistor R2, and the other end of the photo receiver LD is connected to the awakening terminal of the high voltage measuring circuits 11 to 1n. 
When the low voltage system control circuit 30 outputs a low level awakening signal, the light emitting device LE of the isolation devices D1 to Dn emits light. When the light emitting device LE emits light, the photo receiver LD conducts and a high level awakening signal is supplied to the high voltage measuring circuits 11 to 1n. Because the photo receivers LDs are connected to the anodes V1 to Vn of the blocks B1 to Bn, the ground levels of the awakening signals differs. Corresponding to a supply of the awakening signals, the high voltage measuring circuits 11 to 1n cancel the sleep mode and restart the communication with the low voltage system control circuit 30.
However, in the above-described voltage measuring device, for supplying the awakening signals having different ground levels, the isolation devices D1 to Dn are needed corresponding to the number of the blocks B1 to Bn. Therefore, owing to an increase of the number of parts, there is a problem that cost of the voltage measuring device increases, and a packaging size of the voltage measuring device increases.
[Patent Document 1] Japanese Published Patent Application No. 2000-88898
[Patent Document 2] Japanese Published Patent Application No. 2006-42591
Accordingly, an object of the present invention is to provide a voltage measuring device to allow the number of parts to reduce, so that the cost and the package size may be reduced.